electro osmosis.that topics deals with colloids and their one of the colligative properties that is electro kinetic property.under the electrokinetic colligative property of colloids consist 2 properties mainly electrophoresis and elecoosmosis.the electro osmosis have several application properties.the electroosmosis is mainly deals with the charge of colloidal system and their movements opposite charges.electrical double layer theory.

2. COLLOIDS
• “ Finely divided particles of any substances with diameter lying within 10-2000 A⁰
range dispersed in a medium constitute.”
• A colloidal system consist two phases
1.continuous phase or dispersion medium
2.discontinuous phase or dispersed or suspended phase
• Dispersed phase may not necessarily be a solid always .it may be a liquid or even a gas as
well.
• Dispersion medium may be gas or a liquid or even solid.

3. DIFFERENT COLLOIDAL SYSTEMS
Dispersion medium Dispersed phase Examples
Gas Liquid Clouds ,mists, fogs
Gas Solid Smoke, volcanic dust
Liquid Gas Foams ,whipped cream
Liquid Liquid Emulsions ( milk, cod liver oil)
Liquid Solid Starch ,proteins, arsenic sulphide ,gold
Solid Gas Adsorbed or occluded gases
Solid Liquid Jellies, gels, cheese
Solid Solid Coloured precious stones ,rock salt

4. True solutions or molecular solutions:
when a few crystals of a water soluble substances, like sugar or sodium chloride are added to
water ,they dissolve to give a homogenous solution. The particles of a solution are invisible and do not
settle on standing.
Suspension :
if an insoluble substance like barium sulphate or clay or sand ,is added to water the particles are
large enough to visible ,if not to a naked eye at least under a microscope. They settle down standing for
some time.
Colloidal solution:
in between these two extremes, lie the particles which are bigger than molecules but are too small
to be visible even under a most powerful microscope.

5. ELECTRICAL PROPERTIES OF COLLOIDS
• The most important property of colloidal dispersion is that the particles carry an
electric charge.
• All particles in a given colloidal system carry the same charge & dispersion medium
has an opposite and equal charge , the system at a whole being electrically neutral.
• The presence of similar charges on colloidal particles is largely responsible for
stability of system.
• Mutual force of repulsion between similarly charged particles prevent them
coagulation or aggregation when they come closer.

6. • The origin of these charges may be either external or internal
• When the charges are acquired by adsorption of positive or negative ions from the
medium is called external
• Charges arise due to active acidic or basic groups present in the inherent structure
of macromolecule which remain exposed on the surface ,in such a case the origin
is said to be internal.

7. THE ORIGIN OF CHARGES ON COLLOIDAL
PARTICLES
• The origin of charge on colloidal particles has not been completely understood.
1. Frictional electrification : the origin of charge on the colloidal particles may be
due to frictional electrification.it is believed due to rubbing the dispersed phase
particles and with those of dispersion medium result in some type of charge on
colloidal particles of the sol. this view is not satisfactory.
2. Electron capture by colloidal particles: it is believed that the colloidal solutions
prepared by Bredig’s arc dispersion method acquire a charge by electron capture,
this view is not valid in all cases.
3. Dissociation of surface molecule: the dissociation of surface molecule leads to
electric charge on colloidal particles of the sol.

8. • Eg: consider aqueous solution of soap
C₁₅H₃₁COONa C₁₅H₃₁ COOˉ + Na+
here in the cation passes in to the solvent while the anion have a tendency to form
negatively charged aggregates due to weak interaction forces present in the long
hydrocarbon chains. These aggregates are of colloidal dimensions & are negatively charged.
 This is not valid for colloidal solution of non electrolytes such as clay, smoke etc….

9. 4. Presence of some acidic or basic groups in colloidal solution :
Colloidal particles may acquire electric charge due to the presence of certain
acidic or basic groups in colloidal solution.
# protein molecules give rise to formation of colloidal solutions. Thus the
particles of protein sol either have positive or negative charge depending upon the
pH of medium.
but in the case of an iso-electric point the protein will exist in as uncharged
molecules.

10. 5. Selective adsorption of ions from solution :
colloidal adsorb preferentially positive or negative ions present in the
dispersion medium. When two or more ions are present in the dispersion medium
the selective adsorption of ions common to the colloidal particles take place resulting
in the formation of positively charge or negatively charged particle in colloidal
solution.
In simple words ,the electric charge on the colloidal particles essentially originates by
the selective adsorption of ions common to the colloidal particles from the dispersion
medium.
“Commonly believed , the charge on the colloidal particle is due to preferential
adsorption of either positive or negative ions on their surface.”

11. ELECTRICAL DOUBLE LAYER
• Consider a spherical colloidal particle with a negatively charged surface. This
polyelectrolyte will tend to attract the positive ions from the dispersion medium around
it self.
• As a result the macromolecule will soon be surrounded by an ion atmosphere. a region
in which statistical preference for ions of opposite sign.
• Based on this concept Helmholtz develop correct & formulated idea of rigid double layer.
• With more contribution from Gouy,Debye & Huckel a modified theory was proposed to
explain the charge distribution around a macromolecule,
• According to electric double layer theory “ the ions of charge, opposite to that of the
surface concentrate & tend to orient as immobile layer on surface.as a distance from the
surfaces increases mobility of the ions increased & the ion layers become more & more
diffused”

13. • According to modern views “when a solid is in contact with a liquid a double layer
of ions appears at the surface of separation”
• One part of double layer is fixed on the surface of solid. This is known as fixed part
of the double layer.
• The second part consist of a diffuse or a mobile layer of ions which extends into the
liquid phase.
• This layer consists of ions of both the signs but its net charge is equal and opposite
to that on the fixed part of double layer.
• In colloidal system ,the ions carrying the opposite charge of counter ions, that
present more in diffuse portion of the double layer, giving a net opposite charge to
this layer.

14. ZETA POTENTIAL
• The existence of charges of opposite signs on the fixed & diffuse parts of the double
layer leads to the appearance of a difference of potential between the two layers.
This difference of potential between the two layers is known as zeta potential or
electro-kinetic potential.
ζ=
4𝜋η∪
𝐷
D --- dielectric constant of dispersion medium
U --- mobility of colloidal particle
η --- viscosity

16. FLOCCULATION OF COLLOIDAL SOLUTIONS
• The zeta potential decreases when an ion of opposite charge to that of the colloidal
particle is adsorbed. This reduces the mutual repulsion between the similarly charged
colloidal particles.as a result , the colloidal particles easily come closer to one another
to form a bigger aggregates. This phenomena lead to changing colloidal state to
suspended state is known as coagulation or flocculation of colloidal solution.

18. ELECTRO-KINETIC PROPERTIES OF COLLOIDS
• Since the solid particles & the liquid medium carry opposite charges ,when an
electric field, the particles & the liquid will migrate on opposite directions.
• In an experiments particle can move but not the medium --- Electrophoresis.
• Medium can move but not the particles ---- Electro osmosis

19. ELECTRO OSMOSIS
• Electro osmotic flow ( EOF) (synonymous with electro osmosis or electro
endosmosis ).
• The movement of dispersion medium under the influence of an electric field keeping
the disperse phase stationary is called electro osmosis.
• Electric osmosis is one of the electric properties of colloidal system.
• Colloidal system is electrically neutral . Consequently logical to conclude that the
charge of disperse phase must be balanced by an equal and opposite charge on the
dispersion medium.

20. • Here the movement of dispersion medium occurs towards one of the electrodes in
an electric field and solid disperse particle remain stationary.
• Electro osmotic flow is most essential component in chemical separation
techniques.
• Electro osmotic flow can occur in natural unfiltered water as well as buffered
solution.

22. HISTORY
• Electro osmotic flow was first reported in 1809 by F.F.Reuss ( empirical society of
naturalists of Moscow).
• He showed that water could be made to flow through a plug of clay by applying
electric voltage. Clay is composed of closely packed particles of silica & other
minerals & water flow through the narrow spaces between these particles.
• Any combination of electrolyte & an insulating solid would generate electro
osmotic flow.
• Flow speeds are typically only a few millimeter per second.

23. • The electro osmotic flow is caused by Coulomb force , induced by an electric field
on net mobile electric charge in a solution.
• Because the chemical equilibrium between solid surface & an electrolyte solution
typically leads to the interface acquiring a net fixed electrical charge.it mainly
interface the electrical double layer or Debye layer.
• When an electric field is applied to the fluid ( usually via electrode placed at inlets
& outlets ) the net charge in the electrical double layer is induced to move by the
resulting coulomb force. The resulting flow is termed as Electro osmotic flow.

24. WORKING OF ELECTRO OSMOSIS
(1)
• Observing the electro osmosis by a fixing gel material dispersed phase in a U-tube ,
separating two portions the liquid dispersion medium. The whole tube is now filled
with solvent.
• Thickened form of gel cannot move but it can allow the solvent molecule travels
through it.
• When an electric connection is made between the two arms of the U-tube through a
pair of electrodes the solvent is seen to rise in one of the arms.
• If the solvent is water, the water will flow through gel towards the negative pole
because the water acquire positive charge.( porous gel membrane behave as an set of
capillary tubes).
• During the flow of water through the gel there is a preferential adsorption of OHˉ ions
by the walls of the capillaries leaving H+ ions to the charge the water molecular
positively.

26. (2)
• A simple apparatus for studying electro osmosis
• The colloidal system placed on the central compartment A which is separated from
the compartments B & C filled with water, by the dialyzing membranes M & M’.
• The water in the compartments B & C also extends to the side tubes T & T’.
• When a potential difference is applied across the electrodes held close to the
membranes in the compartments B & C.
• If the particle carry positive charge ,the water will have negative charge .then move
towards anode and hence water level rise.
• Or if particle carry negative charge water will move towards the cathode.

28. APPLICATIONS
• Electro osmotic flow commonly used in microfluidic device, soil analysis &
chemical analysis

32. Thank you
by
anand p p